This is gripping on this trigger really that difficult.
Right Thumb on DR
Right Middle on UR
Right Index on UFR
Left Index on BLU
My right hand always lets go before the U'
You don't have to let it completely go. You can just lift the fingers a bit and let the U' slide under the fingers. That depends a little on the following move.
For example: if you use double Sune for double headlights OLL (21):
R U2 R' U'
R U R' U' R U' R', then you can slide the last U' under your fingers because R is next and you have to close that same grip again.
In T-perm there is R' after the last U', then you probably regrip it anyway, so you can let the right hand off the cube completely during the U'.
Additionally pay attention to the other fingers of left hand.
Little finger - nothing
Thumb - place it on the cross of 4 pieces: Center of F, FL, FD, FLD
Middle finger - BL and B center
Ring finger - BLD and BD.
That way you keep the 2x2x3 block fixed.
Another thing is that you can overturn R and R' moves and underturn U and U' turns.
Peedu